A U.S. Navy reconnaissance aircraft and a Chinese military plane were involved in an “unsafe” encounter earlier this week in international airspace over the disputed South China Sea, the U.S. Pacific Command said Friday.

The U.S. Navy P-3C was on a “routine mission” over the waters on Wednesday when the encounter with a Chinese military KJ-200 aircraft occurred, Pacific Command said.

The two planes flew within 1,000 feet (300 meters) of each other in the area of the contested Scarborough Shoal, just 230 km (140 miles) from the Philippine coast, CNN reported, citing unnamed U.S. defense officials.

Scarborough Shoal, which is also claimed by Manila, has been known as a potential flashpoint, and rumors of a push by China to build on the collection of rocky outcroppings have stoked concern in the region. Building at Scarborough would create a large “strategic triangle” covering much of the South China Sea that would give it the ability to declare and police an air defense identification zone (ADIZ) there.

“The Department of Defense and U.S. Pacific Command are always concerned about unsafe interactions with Chinese military forces,” U.S. Pacific Command spokesman Maj. Rob Shuford said in a statement. “We will address the issue in appropriate diplomatic and military channels.”

Chinese state media quoted an unnamed official with the country’s Defense Ministry as saying that the pilot responded with “legal and professional measures.”

“We hope the U.S. side keeps in mind the present condition of relations between the two countries and militaries, and takes concrete measures to eliminate the root causes of air and sea mishaps between the two countries,” the Global Times quoted the official as saying.

In June, a Chinese fighter jet made an “unsafe” intercept of a U.S. spy plane in international airspace over the East China Sea, where Beijing and Tokyo are embroiled in a dispute over the Senkaku Islands, known in China as the Diaoyus.

A month earlier, the Pentagon said two Chinese fighters engaged in an unsafe intercept of a U.S. spy plane in the South China Sea after Washington conducted its third “freedom of navigation” operation in the disputed waters.

Wednesday’s close encounter was believed to be the first since two intercepts in May and June of last year, and is the first such incident under the administration of U.S. President Donald Trump, who has taken a harder line against China than his predecessor, Barack Obama.

The incident is likely to be seen in Washington as a test of the new administration’s mettle.

It came just days ahead of the first phone call between Trump and Chinese President Xi Jinping since the U.S. leader’s inauguration on Jan. 20. Trump had spoken with more than a dozen world leaders before his call with Xi late Thursday, a move some observers said was intentional.

It was not known if the two discussed the South China Sea issue.

How Trump will tackle it remains unknown, though the prospect of a military confrontation over the disputed waters has been raised by Trump’s chief strategist, Steve Bannon.

Bannon said last year that he envisioned the possibility of a U.S.-China war over the strategic waterway within five to 10 years.

China claims most of the waters in the South China Sea, through which $5 trillion in trade passes each year. The Philippines, Vietnam, Malaysia, Taiwan and Brunei also have overlapping claims.

A U.S. think tank said in December that China appears to have built up significant anti-aircraft and anti-missile systems on all seven of its man-made islands in the South China Sea’s Spratly chain.